Thermally expandable microcapsules have been used for various applications as a design-imparting agent or a weight-reducing agent. They have been also used for paint, such as foaming ink and wallpaper, to achieve weight reduction.
As such a thermally expandable microcapsule, widely known is one in which a thermoplastic shell polymer includes a volatile expansion agent that can change to a gas state at a softening temperature or lower of the shell polymer. Patent Document 1, for example, discloses a method for producing a thermally expandable microcapsule that includes a volatile expansion agent by adding an oily mixture and an oil-soluble polymerization catalyst to an aqueous dispersion medium that contains a dispersant with stirring to cause suspension polymerization. Here, the oil mixture is a mixture of a monomer and a volatile expansion agent with a low boiling point, such as an aliphatic hydrocarbon.
Although such a thermally expandable microcapsule obtained by this method is thermally expandable at a relatively low temperature of about 80 to 130° C., the expanded microcapsule bursts or contracts upon heating at a high temperature or a long period of time, and therefore the expansion ratio decreases. Accordingly, this method has a drawback in that a thermally expandable microcapsule having excellent heat resistance is difficult to produce.
Patent Document 2 discloses a method for producing a thermally expandable microcapsule that contains a polymer shell made from a polymerizable component containing 80 to 97% by weight of nitrile monomers, 3 to 20% by weight of non-nitrile monomers, and 0.1 to 1% by weight of a trifunctional crosslinking agent that includes a volatile expansion agent.
Patent Document 3 discloses a thermally expandable microcapsule that includes a volatile expansion agent with a polymer made from a polymerizable component containing 80% by weight or more of nitrile monomers, 20% by weight or less of non-nitrile monomers, and 0.1 to 1% by weight of a crosslinking agent. Here, the non-nitrile monomer is a methacrylic acid ester or acrylic ester.
These patent documents report that, compared with conventional microcapsules, thermally expandable microcapsules obtained by these methods shows excellent heat resistance and do not foam at 140° C. or lower. In fact, when the thermally expandable microcapsules are heated at 130 to 140° C. for about 1 minute, part of them are thermally expanded. Thus, it is difficult to produce thermally expandable microcapsules with excellent heat resistance, such as ones having a maximum foaming temperature of 180° C. or higher.
Patent Document 4 discloses a thermally expandable microcapsule that includes: a shell polymer of a homopolymer or copolymer of ethylenically-unsaturated monomers including 85% by weight or more of a nitrile group-containing monomer; and a foaming agent containing 50% by weight or more of isooctane. The thermally expandable microcapsule is produced to give a maximum foaming temperature of 180° C. or higher and desirably 190° C. or higher.
Although such a thermally expandable microcapsule shows a very high maximum foaming temperature, it is difficult to keep the expanded state. Thus, it is not suitable for prolonged use at a high temperature.
Patent Document 5 discloses a thermally expandable microcapsule that shows good foaming performance and improves in heat resistance in a broad range of foaming temperatures, especially at a high temperature (160° C. or higher) by specifying a monomer that forms the shell of the thermally expandable microcapsule. This thermally expandable microcapsule shows a high maximum foaming temperature. However, upon use in molding processes involving strong shearing force, such as kneading molding, calender molding, extrusion molding, and injection molding, especially for injection molding, deformation may arise because of some problems in heat resistance and strength of the thermally expandable microcapsule in melt kneading. Otherwise, the thermally expandable microcapsule may be crushed.
Patent Document 6 discloses a thermally expandable microcapsule in which a polymer obtainable by polymerizing a carboxyl group-containing monomer and a monomer having a group reactable with a carboxyl group is used as a shell. The patent document reports that such a thermally expandable microcapsule has increased three-dimensional crosslinking density. Therefore, it shows strong resistance to contraction, and marked improvement in heat resistance, with an even when the foamed shell is very thin.
However, such a method still leaves problems of heat resistance and strength. Thus, there is a limitation in the expansion ratios after molding such as injection molding.
Accordingly, it has been required to produce a thermally expandable microcapsule, which shows excellent heat resistance and a high expansion ratio, and is less likely to cause deformation, and is suitably used for molding processes involving strong shearing force, such as kneading molding, calender molding, extrusion molding, and injection molding.
Patent Document 1: Japanese Kokai Publication S42-26524 (JP-A S42-26524)
Patent Document 2: Japanese Kokai Publication H5-15499 (JP-A H5-15499)
Patent Document 3: Japanese Patent No. 2894990
Patent Document 4: EP 1149628
Patent Document 5: WO 2003/099955
Patent Document 6: WO 1999/43758